A photoelectric converter-type pulse encoder of a prior art is illustrated in FIGS. 1 and 2. In FIG. 1, a fixed slit plate 1, a rotating slit plate 2, light-emitting elements 3 and 3', light-receiving elements 4 and 4', a rotational shaft 5, a converter circuit 6, and electric signal lines 7 are shown. Lattice patterns PTN(1) and PTN(2) are formed on the fixed slit plate 1 and the rotating slit plate 2, respectively, as shown in FIG. 2. The lattice pattern PTN(1) has lattice pattern rows PA, PA, PB, and PB, and the lattice pattern PTN(2) has a lattice pattern row PC.
The light-emitting element 3 emits light through the pattern row PC of the rotating slit plate 2 and the pattern rows PA and PA of the fixed slit plate 1 to the light-receiving element 4 while the light-emitting element 3' emits light through the pattern row PC and the pattern rows PB and PB to the light-receiving element 4'. In the light-receiving elements 4 and 4', electric signals are produced in response to the intensity of the received light and are supplied to the converter circuit 6 through the electric signal lines 7.
The rotating slit plate 2 is able to rotate around the rotational shaft 5, and the amount of light emitted from the light-emitting elements 3 and 3' to the light-receiving elements 4 and 4' varies in response to the difference between the positions of the lattice patterns of the fixed slit plate and the rotating slit plate in accordance with the rotation of the rotating slit plate 2. Thus, the electric signals produced in the light-receiving elements 4 and 4' are altered sinusoidally.
In FIGS. 3 and 4, various signal waveforms are illustrated in order to explain the operation of the pulse encoder of FIG. 1. In the pulse encoder of FIG. 1, it is necessary to gain two rectangular pulse signals S.sub.A and S.sub.B, the phase difference between these signals being 90.degree. and the duty ratio between these signals being 1. However, it is difficult to gain the above-mentioned two pulse signals only by comparing two sine wave signals with a 90.degree. phase difference to a reference value since it is extremely difficult to control the DC components of the signals obtained by the light-receiving elements 3 and 3' at constant values.
In order to solve the above-mentioned problem, in the pulse encoder of FIG. 1, a method is used in which, in order to gain a pulse signal S.sub.A of phase A, the two lattice pattern rows PA and PA are provided and two signals CA and CA with inverted phases are produced. These two signals CA and CA are adjusted so that they have the same DC components. Then these two signals are compared so that the pulse signal S.sub.A is produced. FIGS. 4(1) and 4(2) illustrate that a rectangular pulse signal with a duty cycle of 50% can be obtained by the above-mentioned method.
However, in the above-mentioned method, considerably troublesome adjustments are necessary in order to make the DC components of the two signals equal. That is, in order to match the light axes between the light-emitting elements and the light-receiving elements so as to equalize the distribution of light between the light-emitting elements and the light receiving elements, it is necessary to carry out the following adjustments:
(1) adjustment of the position of the light-emitting element so that the center of light distribution is on the borderline between the two lattice plates PA and PA,
(2) adjustment of the inclination of the light-emitting elements so as to equalize the distribution of light,
(3) adjustment of the light distribution by the provision of a blind in front of the light-emitting elements so as to eliminate excess light.
The above-mentioned adjustments are very difficult to carry out, and, furthermore, it is very difficult to maintain these adjustments.